Always reset null templatefields to an empty array instead

[rossriley]

Fixes #6790

Because we don't send null values to the database to save, we end up not persisting templatefields if all the values are cleared.

This ensures that a null templatefields is converted to an empty array instead and then will get saved to the db.

[stueybrock]

Thanks for looking into this so quickly @rossriley !

$value is never null - it's an instance of Bolt\Storage\Entity\TemplateFields or an array of values. The empty value is an array:

[
	0 => ''
]

This produces an output of:

{
    "funders_section_2" : "[\"\"]"
}

If you then refresh the page, you get the following error:

However if you change the value in the DB to:

{
    "funders_section_2" : "[{\"\"}]"
}

This works straight away. I presume that repeaters are looking for an subarray which the first instance doesn't produce?